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Modeling and Testing an 802.11ax RF Transmitter

This example shows how to characterize the impact of RF impairments in an 802.11ax transmitter. The example generates a baseband IEEE® 802.11ax™ waveform by using WLAN Toolbox™ and models the RF transmitter by using RF Blockset™.

Introduction

This example characterizes the impact of RF impairments such as in-phase and quadrature (IQ) imbalance, phase noise, and power amplifier (PA) nonlinearities on the transmission of an 802.11ax waveform. To evaluate the impact of these impairments, the example performs these measurements:

  • Error vector magnitude (EVM): vector difference at a given time between the ideal (transmitted) signal and the measured (received) signal

  • Spectral mask: test that ensures a transmission in one channel does not cause substantial interference in adjacent channels

  • Occupied bandwidth: bandwidth that contains 99% of the total integrated power of the signal, centered on the assigned channel frequency

  • Channel power: filtered mean power centered on the assigned channel frequency

  • Complementary cumulative distribution function (CCDF): probability that the signal's instantaneous power is at a specified level above its average power

  • Peak-to-average power ratio (PAPR): relation between the peak power of the signal and its average power

The example works on a packet-by-packet basis and uses a Simulink model to perform these steps:

  1. Generate the baseband 802.11ax waveform by using WLAN Toolbox features.

  2. Oversample and filter the waveform by using an FIR Interpolation block.

  3. Import the baseband waveform into the RF Transmitter Subsystem block implemented by using RF Blockset blocks. The model uses an RF intermediate frequency to carry the baseband information in RF Blockset.

  4. Model the effects of upconverting the waveform to the carrier frequency by using the RF Transmitter Subsystem block. This block models the impairments introduced by an RF transmitter using RF Blockset blocks.

  5. Calculate the occupied bandwidth and channel power and depict the spectral mask by using the Spectrum Analyzer block.

  6. Compute the CCDF and PAPR by using the CCDF and PAPR block.

  7. Downsample and filter the waveform by using an FIR Decimation block.

  8. Extract the data symbols and measure the EVM by demodulating the baseband waveform.

The Simulink model uses WLAN Toolbox and DSP System Toolbox™ features to process the baseband signal (steps 1, 2, and 5-8) and uses RF Blockset blocks to model the RF transmitter (steps 3 and 4). This model supports Normal and Accelerator simulation modes.

Simulink Model Structure

The model contains three main parts:

  • Baseband Waveform Generation: generates the baseband 802.11ax waveforms

  • RF Transmission: models the effects of upconverting the waveform to the carrier frequency

  • Baseband Waveform Reception: performs the RF measurements and calculates EVM by demodulating the baseband waveform

modelName = 'HERFTransmitterModel';
open_system(modelName);

Baseband Waveform Generation

The WLAN 802.11ax block generates standard-compliant high-efficiency single-user (HE SU) waveforms, according to IEEE P802.11ax/D7.0. You can generate this block using the WLAN Waveform Generator app. You can access the waveform configuration parameters in the user data of the block. This example uses the InitFcn in the Model callbacks to store the structure available in the user data in a Base Workspace variable, HEInfo. For more information about this block, see Waveform From Wireless Waveform Generator App.

To display the effect of the high-power amplifier (HPA) on the out-of-band spectral emissions, the FIR Interpolation block oversamples and filters the waveform. At the output of the RF Transmitter Subsystem block, the FIR Decimation block downsamples the waveform back to its original sampling rate. The Multirate Parameters block provides an interface to configure the parameters of the FIR Interpolation and Decimation blocks.

Specifying Simulation Time

Set the Stop Time value in the Simulink model to the time required to transmit and obtain the EVM results and constellation diagram of, at least, one packet. Considering the waveform configuration selected in the WLAN 802.11ax block, the packet transmission time in this example is 304.4 microseconds. As the filters in the FIR Interpolation and Decimation blocks introduce a delay, you can use the IdleTime parameter in the Mask Editor of the WLAN 802.11ax block to compensate for the delay.

RF Transmission

The RF Transmitter Subsystem block is based on a superheterodyne transmitter architecture. This architecture models the effects of upconverting the waveform to the carrier frequency by characterizing these RF components:

  • IQ modulator consisting of mixers, a phase shifter, and a local oscillator

  • Bandpass filter

  • Power amplifier

In addition to these components, this RF Transmitter Subsystem block also includes a variable gain amplifier (VGA) to control the input back-off (IBO) level of the HPA.

set_param(modelName,'Open','off');
RFTransmitterBlock = [modelName '/RF Transmitter'];
set_param(RFTransmitterBlock,'Open','on');

Use an Input Buffer block to send one sample at a time to the RF Transmitter Subsystem block.

The Inport block inside the RF Transmitter Subsystem block converts the Simulink complex baseband waveform into the RF Blockset Circuit Envelope simulation environment. The Carrier frequencies parameter of the Inport block specifies the center frequency of the carrier in the RF Blockset domain. The Outport block converts the RF Blockset signal back into Simulink complex baseband.

You can configure the RF Transmitter components by using the RF Transmitter Subsystem block mask.

The RF Transmitter Subsystem block models typical impairments, including:

  • I/Q imbalance as a result of gain or phase mismatches between the parallel sections of the transmitter chain dealing with the IQ signal paths

  • Phase noise as an effect directly related to the thermal noise within the active devices of the oscillator

  • PA nonlinearities due to DC power limitation when the amplifier works in the saturation region

Adapt the power level of the baseband waveform to the RF configuration by adding a Gain Control block after the Input Buffer block.

As the current RF Transmitter Subsystem block configuration sends one sample at a time, the Output Buffer block (after the RF Transmitter Subsystem block) collects all samples within an HE SU packet before sending the samples onto the Demodulation and EVM calculation block.

Baseband Waveform Reception

The Demodulation and EVM calculation block recovers and plots the HE Data symbols in the Constellation Diagram block by performing frequency and packet offset corrections, channel estimation, pilot phase tracking, OFDM demodulation, and equalization. This block also performs these EVM measurements:

  • EVM per subcarrier (dB): EVM averaged over the allocated HE Data symbols within a subcarrier

  • EVM per OFDM symbol (dB)

  • Overall EVM (dB and %): EVM averaged over all transmitted HE Data symbols

The Spectrum Analyzer block depicts the spectral mask according to IEEE P802.11ax/D7.0 Section 27.3.19.1. A second Spectrum Analyzer block, called CCDF and PAPR, connected at the input of the HPA block depicts the CCDF and PAPR measurements. The Power Meter block measures the RF waveform channel power, which is displayed in the Output Power (dBm) block.

Effect of Power Amplifier Nonlinearities

To characterize the impact of HPA nonlinearities in the EVM evaluation, you can measure the amplitude-to-amplitude modulation (AM/AM) of the HPA. The AM/AM refers to the output power levels in terms of the input power levels. The helper function hePlotHPACurve displays the AM/AM characteristic of the HPA selected for this model.

hePlotHPACurve();
figHPA = gcf;

P1dB is the power at 1 dB compression point and is normally used as a reference when selecting the IBO level of the HPA. You can see the HPA impact on the RF Transmitter Subsystem block by analyzing the EVM results for different operating points of the HPA. For example, compare the case when IBO = 11 dB, corresponding to HPA operating in the linear region, with the case when IBO = 3 dB, corresponding to HPA operating in saturation. The gain of the VGA controls the IBO level. To keep a VGA linear behavior using the default parameters, select gain values lower than 15 dB.

  • Linear HPA (IBO = 11 dB). To operate at an IBO level of 11 dB, set the Available power gain parameter of the VGA block to 5 dB. To calculate the EVM and plot the constellation diagram, run the simulation to capture one packet (Stop Time equal to 304.4 us for the default configuration).

set_param(RFTransmitterBlock,'vgaGain','5');
sim(modelName);

According to IEEE P802.11ax/D7.0 Table 27-49, the allowed relative constellation error (EVM) in an HE SU PPDU when the dual carrier modulation, or DCM parameter in the WLAN 802.11ax block Mask Editor, is equal to false and the Modulation/coding, or MCS parameter in the WLAN 802.11ax block Mask Editor, is equal to 3 (16-QAM, 1/2) is -16 dB. As the overall EVM, around -41 dB, is lower than -16 dB, this architecture falls within the requirements of IEEE P802.11ax/D7.0.

  • Nonlinear HPA (IBO = 3 dB). To operate at an IBO level of 3 dB, set the Available power gain parameter of the VGA block to 13 dB.

set_param(RFTransmitterBlock,'vgaGain','13');
sim(modelName);

% Restore to default parameters
set_param(RFTransmitterBlock,'vgaGain','5');

Compared to the previous case, the constellation diagram is more distorted. In terms of measurements, the overall EVM, around -28 dB, is still lower than -16 dB, so it also falls within the requirements of IEEE P802.11ax/D7.0.

Summary and Further Exploration

This example demonstrates how to model and test the transmission of an 802.11ax waveform. The RF Transmitter Subsystem block consists of a bandpass filter, amplifiers and an IQ modulator. The example highlights the effect of HPA nonlinearities on the performance of the RF Transmitter Subsystem block. You can explore the impact of altering other impairments as well. For example:

  • Increase I/Q imbalance by using the I/Q gain mismatch (dB) and I/Q phase mismatch (Deg) parameters on the IQ Modulator tab of the RF Transmitter Subsystem block.

  • Increase the phase noise by using Phase noise offset (Hz) and Phase noise level (dBc/Hz) parameters on the IQ Modulator tab of the RF Transmitter Subsystem block.

The RF Transmitter Subsystem block is configured to work with the current HE waveform parameters selected in the WLAN 802.11ax block and with the RF carrier centered at 5950 MHz. This carrier is within the IEEE 802.11 HE STA frequency bands (between 1 GHz and 7.125 GHz, according to IEEE P802.11ax/D7.0). If you modify the Center frequency (MHz) parameter of the RF Transmitter Subsystem block or the waveform configuration of the WLAN 802.11ax block, check if you need to update the parameters of the RF Transmitter components and the FIR filters as these parameters are set to work with the current example configuration. For instance, a change in the carrier frequency requires revising the Passband frequencies and Stopband frequencies parameters of the Bandpass Filter block inside the RF Transmitter. If you increase the waveform bandwidth, check if you need to update the Impulse response duration and Phase noise frequency offset (Hz) parameters of the IQ Modulator (RF Blockset) block. The phase noise offset determines the lower limit of the impulse response duration. If the phase noise frequency offset resolution is high for a given impulse response duration, a warning message appears, specifying the minimum duration suitable for the required resolution.

You can use this example as the basis for testing HE waveforms for different RF configurations. You can replace the RF Transmitter Subsystem block by another RF subsystem and then configure the model accordingly.

To use a different HE SU waveform, open the WLAN Waveform Generator app, select the HE SU configuration, and export a new block. For more information on how to generate and use this block, see Generate Wireless Waveform in Simulink Using App-Generated Block.

References

  1. IEEE P802.11ax™/D7.0 Draft Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications - Amendment 6: Enhancements for High Efficiency WLAN.